Choosing your subwoofer

I chose an Alpine 10" subwoofer for my CR-V because it requires relatively little box volume.

When choosing a subwoofer, carefully note the manufacturer's recommended enclosure size, especially if you have limited space in your vehicle. The box volume tells you how big your enclosure needs to be for maximum performance from the sub. The box shown in this article was intended for my Honda CR-V, so I wanted to make sure it wouldn't take up too much space. With that requirement in mind, I chose a 10" Alpine sub. The manufacturer recommends a box volume of 0.6 to 1.25 cubic feet.

Designing the box

To determine the correct dimensions for any subwoofer box, you can follow these steps:

Determine the minimum depth of your box. Measure the depth of your subwoofer and add 2 inches. This measurement is the minimum depth of your box (in this article, the depth refers to the front-to-back dimension of the box, with the woofer being mounted to the front).

Determine the minimum height and width of your box. Measure the frame diameter of your woofer, or check the mounting template that may be included with the owner's manual, to determine the minimum height and width for the front of your box. If you plan to mount a grille, be sure to allow for any additional space that may be needed to accommodate it.

Determine the available space in your vehicle. Measure the height, width, and depth of the vehicle space that you are willing to devote to your subwoofer. If the box must be wedge-shaped to fit, you will need to know the depth at the box's top and bottom.

Sketch out your box. Now's the time to sketch your box on paper using the dimensions you've gathered. The box shown in this article is rectangular, but you may find that a wedge-shaped box fits better in your car or truck. Your sketch may look like one of these:

For our examples, let's use the following external dimensions:

Rectangular box:
Height: 13"
Width: 14"
Depth: 12"

Wedge box:
Height: 14"
Width: 18"
Depth 1: 5"
Depth 2: 8"

Determine the internal dimensions and volume of your box. The above steps identified the external dimensions of the box. To determine the internal volume, just subtract the thickness of the wood to be used for construction. If you're using 3/4" MDF (recommended!), then 2 x 3/4", or 1-1/2", will be subtracted from each dimension.

The internal dimensions for our examples:

Rectangular box:
Height: 11.5"
Width: 12.5"
Depth: 10.5"

Wedge box:
Height: 12.5"
Width: 16.5"
Depth 1: 3.5"
Depth 2: 6.5"

Calculate the internal box volume in cubic inches. Based on the internal dimensions, you can calculate the internal volume of the enclosure using the following formula: Height x Width x Depth = Cubic Volume

Let's plug in some numbers:

Rectangular box:

11.5" x 12.5" x 10.5" = 1,509.375 cubic inches

Wedge box: Since the wedge box has two depth dimensions, we need to find the average depth before we can determine the volume. To find the average depth, add the two depth measurements together, then divide by two. Remember that Depth 1 = 3.5 and Depth 2 = 6.5.

3.5" + 6.5" = 10"
10" / 2 = 5"

So, the average depth of the wedge-shaped box is 5". Plug that dimension into the formula:

12.5" x 16.5" x 5" = 1,031.25 cubic inches

Convert cubic inches to cubic feet. Since most manufacturers will provide the recommended box volume in cubic feet, you'll need to convert the internal volume from cubic inches into cubic feet. This is done by dividing the cubic inches by 1,728.

Rectangular box: 1,509.375 / 1728 = 0.873 cubic feet

Wedge box: 1,031.25 / 1728 = 0.597 cubic feet

Adjust your box's volume to match the sub's specifications. Now, compare the volume of the box you've sketched to the manufacturer's recommendation. If it's too large or too small, you can make small adjustments to one dimension until your box's internal volume matches the manufacturer's recommendation as closely as possible. Often, manufacturers will recommend a range of enclosure volumes. You can get good results with a box that's anywhere inside the recommended range.

Determine the final exterior box dimensions. Once you've identified the correct internal dimensions, it's time to add back that 1-1/2" we subtracted in step 5, to derive the new external dimensions. Double check to make sure that these dimensions will fit properly in your car, and you're ready to move on to construction.

Step-by-step instructions

Click on the photos to enlarge.

Step 1.
We started by measuring and cutting the main pieces of MDF for the front, sides, back and top of the box, using a table saw with a carbide-tipped blade.

Don't have a table saw? Don't worry. If you purchase your MDF at any large home improvement store, they should be able to cut it for you for a small fee. You could also use a jigsaw, but your cuts may not be quite as smooth (and smooth, flat cuts help to ensure that the box seals well).

We cut seven pieces total — top and bottom, two sides, the back, and two identical pieces for the front (since it was to be a double thickness).

Step 2.
After the pieces are cut to size, use a compass, or the template that may be included in your subwoofer's packaging, to mark the woofer cutout on one of the identical front pieces.

Step 3.
If you choose to use a double-thickness of MDF for the front panel (this method is recommended — it provides an extremely strong, non-resonant mounting surface for the sub), fasten the two identical front pieces together using plenty of carpenter's glue and several sheet metal screws. Also, the double thickness will serve to strengthen the box as a whole.

(If you don't use a double thickness of MDF for the mounting surface, you should definitely plan to use bracing elsewhere in the box for added strength. In fact, it's never a bad idea to use bracing no matter what, especially if your box is larger than a cubic foot. The box will be subjected to extreme internal pressure, so the stronger it is, the better. The easiest way to add bracing is with 2"x2" strips of lumber. Glue and screw these along at least two of your box's internal seams before attaching the top and bottom.)

Step 4.
Using a drillpress, we made a hole near the inside edge of the circle we had traced, large enough for our jigsaw blade to fit in. If you don't have a drillpress, simply use your handheld drill and a large bit.

We jigsawed out the circle, and the woofer cutout was complete.

Step 5.
We followed the same drilling/jigsawing procedures to make a rectangular hole in the box's back panel. This would hold the terminal cup, which we installed next.

After running a bead of silicon caulk around the edge of the terminal cup (available from Crutchfield, item number # 0732531034, $4.00), we screwed it into place using 1/2" sheet metal screws.

Step 6.
Since the back and front pieces were now complete, it was time to fasten everything together. Note: the largest sides of the box should overlap each of the smaller sides to provide the greatest strength. For our box, that meant that the sides were fastened to the front and back first, and the top and bottom were added last.
Because MDF can be prone to splitting, we pre-drilled holes for the screws in each of the pieces to be fastened together.

After pre-drilling the holes, we squeezed plenty of carpenter's glue between the pieces. The glue, not the screws, is what will ultimately seal the box together, so don't be afraid to pour it on.

Then, we fastened the pieces together using our cordless drill and 2" drywall screws. Some of the glue will squeeze out during this step — you can wipe it off the outside of the box using a wet rag, but it's OK to leave it on the inside edges (it'll actually help with the seal).

After you put together the front, back, and sides, you may find that the box is a little out of square — we did. When you screw the top or bottom on, it should pull things back into alignment. Ours gave us just a little bit of trouble, though, so we used a furniture clamp to get things straightened out.

After you've glued-and-screwed the sides, front, back, top, and bottom, you're just about done.

The next step is to drop the subwoofer in and make sure it fits. If the box has gotten a bit out of square, you may find that the sub is now a tight fit — if so, use coarse sandpaper or a rasp to enlarge the opening a little.

Step 7.
With the subwoofer in place, we used a pencil to mark the screw hole locations, then removed the subwoofer and pre-drilled holes for the mounting screws.

Step 8.
To make double sure that everything was sealed up, we waited for the glue to dry and ran a bead of silicon caulk over all of the box's internal seals. We used a hand-held tube of caulk, since it would be tough to get a caulk gun down inside the box at this point. Let the caulk cure for 12-24 hours before putting the subwoofer back in. Some silicon caulk releases acetic acid fumes while curing, which have the potential to destroy subwoofer surrounds.

Step 9.
After the caulk had cured, we hooked up speaker wires from the terminal cup to the subwoofer and placed the subwoofer back in the box, using non-hardening rope caulk (found in the weatherstrip section of the hardware store) to seal it down.

Finishing and customizing the subwoofer box

After finishing the box, I couldn't wait to pop it in my CR-V. After the glue and caulk had dried, we hooked it up to the rear channels of my Alpine 4-channel amplifier using 12-gauge speaker cable. It sounded excellent, and the box was definitely sturdy.

However, since naked MDF leaves something to be desired in the looks department, I checked in with Crutchfield's installation specialist, Warren Hawkins, to get some ideas on customizing a subwoofer box. He offered up plenty of inspiration and installation tips on all kinds of coverings, from vinyl to carpet and beyond!

Thanks to Kelley Blanton, and to Taylor and Boody Organbuilders of Staunton, VA, for the use of their facility and tools.

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Crutchfield Writer Biography

Erin Blanton

Erin Blanton wrote about mobile A/V gear at Crutchfield for several years, and worked as a Sales Advisor in our contact center before that. She left Crutchfield to open her own toy store, Pufferbellies, in nearby Staunton, Virginia, which continues to keep her busy.